Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

EFFECT OF PEESSUEE ON BOILING POINT 25
tion lowers the boiling point can be seen in Fig. 20, in which nitrobenzene,
boiling point 208°/760 mm. (curve I), and benzaldehyde (II),
boiling point 179°/760 mm., are the examples chosen. The importance
of a " good vacuum " in preparative work is expressed in the steep
slope of the curves in the low-pressure region. When the distillation is
carried out at 20 mm. the boiling point is about 15° higher than when
the pressure is 10 mm. As the pressure rises this effect is reduced, as
the curve III in the upper part of the figure makes clear. This curve,
mmHg 20 3 2 10 BoiimB
Point
which is on a different scale, represents the region of pressure from
760 mm. downwards. At Munich a reduction of the sea-level barometer
to 720 mm. only lowers the boiling point of water by 1-5°.
The quantitative relations between pressure and boiling point
vary from substance to substance, but among organic compounds
the variation is not very great, so that the curves here reproduced
can in practice very well be used as a guide.
For example, if according to the literature a substance boils at
96°/l2 mm., it will boil at 104-105° at 18 mm.
Substances which boil at too high a temperature, even in the vacuum
of the water pump, can often be distilled without decomposition in a
high vacuum, i.e. at pressures of 1 mm. or less. Reduction of pressure